It is known that iron-manganese-aluminum alloys can provide steels with austenitic structure, having the desirable characteristics of low density, resistance to oxidation, and high strength plus superior cold ductility for ready formability and toughness in service. Iron-manganese-aluminum alloys including small quantities of additional alloying elements are described in U.S. Pat. Nos. 3,111,405 (Cairns et al.) and 3,193,384 (Richardson).
However, the production of alloys of this general character having suitable properties and hot-workability to allow economical manufacture on conventional steel mill facilities requires control of the resulting cast alloy crystal structure, i.e. the relative proportions of body-centered (ferritic) crystal structure and face-centered (austenitic) crystal structure in the alloy must be present within a specified range to ensure that the alloys can be hot rolled with good yield to a useful product. These alloys are expected to find application primarily in plate, sheet and strip form. The hot rolling of these product forms makes this control of the proportions of ferrite and austenite particularly critical, owing to the high speeds and high rates of deformation encountered in commercial mill operations.
The ferrite-austenite ratio in austenitic steel alloys is of critical importance to the final properties of a steel alloy, and is itself dependent upon the elemental composition of the alloy. Thus, while a high aluminum content is desirable in these steel alloys to impart both superior oxidation resistance and a lower density, the aluminum concentrations required, in order to contribute significantly to those objectives, tend to result in a ferritic structure that is not readily hot-worked by conventional methods to produce marketable products. Further, a high aluminum steel product may exhibit limited formability, so that its usefulness in fabricating engineering structures is limited. It is known that the addition of manganese and carbon compensates for these inadequacies of aluminum and promotes the conversion of the ferritic structure to an austenitic structure, resulting in superior hot workability at conventional hot rolling temperatures, as well as ensuring the improved qualities of formability, ductility, and toughness arising from the austenitic structure.
Early investigations of iron-manganese-aluminum alloys have recognized the enhancement of properties that can be achieved by increasing the proportion of austenite structure in such products, providing recipes for such alloys but no indication as to how the ferrite-austenite ratio may be controlled by judicious selection of the elemental composition.
S. K. Banerji in his publication "An Update on Fe-Mn-Al Steels", 11 June, 1981 disclosed a useful alloy composition 30% Mn, 9% Al, 1% Si, 1% C, the balance Fe, but has not taught any range of useful alloy compositions encompassing the foregoing, nor any useful quantitative relationship between volume percent ferrite and the element percentage values selected, nor any preferred range of volume percent ferrite.
The applicants have found that precise control of the ratio of the ferritic volume to austenitic volume is critical to the successful hot rolling of iron-manganese-aluminum alloys. It has been found that a maximum of about 8 percent of the ferrite crystal structure form is compatible with economical and efficient hot rolling of the alloy. A level of ferrite in excess of this proportion causes the workpiece to develop surface tears and "pulls", usually requiring scrapping of the product. Hertofore, the problems presented by an alloy composition having too great a proportion of ferrite structure have been addressed by the use of decreased hot rolling temperatures, but that solution comes only at the expense of increased rolling costs and rolling loads on the mill equipment. Further, the hot rolling temperature limits the final minimum size or thickness of the hot rolled product, so that with higher ferrite alloys additional cold reductions are required to obtain the requisite product sizes, with concomitant added cost and complexity in the production process.
On the other hand, if an iron-manganese-aluminum alloy having purely austenitic crystal structure forms during the solidification of a cast ingot or slab, the casting has been found to result in the development of enlarged grains during the solidification process. Again, the consequence is poor hot workability. During hot rolling, the edges of the workpiece develop irregular tears and fissures to a degree that severe edge loss is encountered in the coil or sheet, resulting in costly yield loss and in strips, sheets or coils too narrow for the intended market. For this reason, a number of hitherto available austenitic steels having too low a ferrite crystal structure have been unamenable to the modern and cost-beneficial process of continuous casting of slabs.
Attempts have been made to remedy the problems resulting from too little ferrite by extraordinary control of the casting temperature and/or lower rolling temperatures to minimize the grain size of the casting and the enlargement of the grains during heating for rolling. However, as a practical matter, such extraordinary control requirements are seriously detrimental to good productivity and, even at best, have proved only marginally successful in preventing yield losses and offsize product.